The dpi and ppi values are meaningless as far as resolution goes. Resolution is determined by the lens, the camera sensor, and the aperture of the lens.

Camera sensors have a width and a height in pixels. The more pixels the better the potential resolution until diffraction sets in.

Think of a sheet of paper that has lines on it. The lines are the same width as the space between them. The different sets of lines and spaces vary from wide to narrow. The US Air Force produced such a resolution test chart.

If you take a photo of that sheet of paper the better the lens/sensor combination then you will be able to see some of the sets of individual lines but some of these that are most narrow will blur together. The narrower the lines that you can see the greater the resolution is.

Each lens is different and the resolution of any lens varies with aperture. The resolution for any lens is usually being best at about 2 or 3 stops smaller than the widest aperture and then it starts to decrease again at about f/16.

dpi refers to the dots per inch for a print. It has nothing to do with resolution of an image until you make a print. For an 8"x10" you want to print at about 240-300 dpi so that the dots will not be seen at normal viewing distances. For larger prints the viewing distance is larger so you can print at lower dpi values. For a 20"x30" print, which would normally be viewed at about 36", you only need about 1/2 the dpi value.

ppi referrers to the pixels per inch of your computer monitor. 72 was the normal for CRT monitors but todays LCD monitors usually have a higher dpi value. If you set the monitor dpi value as the default dpi value then you will be roughly seeing a 1:1 magnified image at the correct physical size.

Don gave you an explanation of lens resolution (how well a lens can "resolve" fine detail), but that's not what you were asking about, I don't believe.

It may not be technically correct, but "resolution" is the pixel dimensions. A low resolution image, like one you would post online or in an email, would be 600 x 400 pixels, for example. A high resolution image would be 6000 x 4000, suitable to make a good sized print. If you printed the 6000 x 4000 pixel image at 300 ppi, you would get a 20 x 13.3 inch print (6000/300 by 4000/300). You can change the size of the print using the 6000 x 4000 pixel image by using a different ppi. For example, that same image printed at 200 ppi would give you a 30 x 20 inch print (6000/200 by 4000/200). If you wanted to print that image 30 x 20 inches and use 300 ppi you would have to "resample" in Photoshop to a 9000 x 6000 pixel image. To get that 600 x 400 pixel image suitable for emailing, you would also have to "resample" but downward. In this case, the ppi doesn't matter as only pixel dimensions determine image size on a monitor. (PPI is only important for printing.)

I thought about trying to explain it but the experts do a better job in videos.
Please follow this link to Adobe TV and you will find several videos covering the subject of Image Resizing etc.Image size | Adobe TV

Julianne Kost does a great job in the video, "Understanding Resize vs Resample."

Watch several and I think it will sink in...it takes a bit of brain stretching and at times, I still get confused.

Just remember one Important thing. When resizing DOWN with resample ON you are throwing away pixels and resizing UP you are allowing Adobe to ADD pixels to the image. So the moral is to never use an original image, always use a DNG or TIF copy before you start doing destructive editing. Always preserve your originals! Never use originals for editing.

Enjoy!

P.S. If you need to know what a "pixel" is you can find a great definition in Wikipedia.

Resolution refers to a file's size measured in total pixels. For instance a Canon 6D shoots at a Resolution of 20MP or 5472px x 3648px. Giving the client the file with all those pixels is considered a high resolution file.

Cropping a file to any size and setting the PPi at a specific number (like 300) will reduce the overall pixels (resolution) and eliminate the High Res file status.
Cropping a file to any size and leaving the PPi BLANK will leave all pixels intact, keeping it a High res file. (you're really doing nothing)

Quite a few people have done an excellent job of getting across helpful information to you but if you are still confused I will have ago.
The business of resolution, PPI, DPI, LPcm etc are a direct part of my daily work. I am heading for retirement and I have been writing up notes and designing graphics to pass on to successors. After I had done my notes on PPI and DPI I discovered a contact of mine Owen Ransen had done a much better job than I on his web page: DPI, Dots Per Inch, explained.Megapixels and Inches Explained

Owen leaves off at the point of rescaling (the PS fraternity use the misleading term resampling). In fact most of the time we view the image in hardware that sidesteps the resolution and the DPI/PPI associated with the image, they are merely treated as guidelines. The images are rescaled to fit the viewing technology, be it your camera's LCD screen, computer monitor or printer or whatever. I won't go into the maths here (unless you ask for it) but mathematical processes like weighed averages and interpolation are used to fit the image to the display. They allow a high resolution image to be displayed on a lower resolution 1920 x 1080 LCD and then printed at an apparently higher resolution than the original camera image. Usually the various rescaling ratios are non-integer and that makes it difficult for some people to visualize.
I should point out though that any resolution decrease, over the original camera image, looses information and any increase, over the original camera image, just invents information by a mathematical "best guess". The incredible computer zooms in surveillance images that you see in TV crime shows is mostly fiction.

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